1. Field of the Invention
This invention pertains generally to imaging devices, and more particularly to imaging cameras for detecting ultrahigh energy cosmic rays in space.
2. Description of Related Art
One of the most challenging issues in Astroparticle Physics has been the observation of Extreme Energy Cosmic Rays (EECRs). The existence of particles with energy above ˜5×1019 eV raises fundamental questions about their origin and propagation in space. EECRs have been detected through the Extensive Air Showers (EAS) produced in the Earth Atmosphere. The shower development is accompanied by the emission of fluorescence light in the atmosphere, and in particular, fluorescence light induced in nitrogen with characteristic spectral lines in the ultraviolet (UV) waveband between approximately 300 nm and approximately 400 nm.
Current knowledge of the EECR phenomenon is based on the data of very limited statistics. Planned efforts for the future study of EECRs include ground-based experiments, and experiments in space. Space borne experiments, and in particular the OSS mission Wide-angle Orbiting Lens (OWL), should observe the fluorescence signal in the atmosphere, looking downward from Space at the dark Earth's atmosphere, under ˜60 degree field of view. The fine segmentation and the time resolution of a large camera can allow the reconstruction of the shower arrival direction and energy.
The Extreme Universe Space Observatory (EUSO) is a currently considered space mission that is devoted to the investigation of cosmic rays and downward from Space at the dark Earth's atmosphere, under a 60 degree full filed of view. The fine segmentation and the time resolution of a large camera can allow the reconstruction of the shower arrival direction and energy. EUSO is expected to detect of the order of 103 EECRs of energy E>1020 eV per year, and to open a window into the High Energy neutrino Universe. Current ideas for the design of the camera are essentially based on classical photomultiplier tubes, or hybrid photon detectors. Apart from being heavy and complicated for space applications, such cameras can have other problems, like low quantum efficiency, in particular in the important ultraviolet region, very large dead area around sensitive pixels, non-uniform response, and high cost.
A small number of cosmic rays of extreme energies have been discovered in ground-based experiments. The origin of these cosmic rays presents a complete mystery, since they should not be able to arrive from large distances (due to the interaction with the low-energy photon background), and on the other hand there is no candidate for the “accelerator” of such particles nearby, or in fact, anywhere. A possible solution of this mystery can have a very strong impact on cosmology. The large ground-based experiment “Auger” is dedicated to study this phenomenon. However, much higher event statistics can be achieved by measurements from space. In order to provide for imaging in space, however, a very large (e.g., 3 m in diameter) and very low weight imaging camera is needed.
Current camera designs are based on a very large number (>10,000) of classical photomultiplier tubes (PMT) arranged in the focal surface of a large Fresnel lens system. Although in principle feasible, this concept is hampered by numerous weaknesses. Apart from the large mass and immense constructional complexity, such cameras can have an important scientific drawback, e.g., low sensitivity, and consequently relatively high detection threshold energy, and poor energy resolution. Other drawbacks include very low quantum efficiency (in particular in the important ultraviolet region), very large dead area around the sensitive pixels, and very high cost. It happens that PMTs present the main source of these problems.
For example, FIG. 1 is a schematic diagram of an imaging electron lens, generally designated 10, that uses a transmission-mode photocathode according to a design which is described in D. Ferenc, Imaging Hybrid Photon Detectors with Minimized Dead Area and Protection Against Positive Ion Feedback, Nucl. Instr. and Meth. A431(1999) pp. 460-475, incorporated herein by reference.
As shown in FIG. 1, the electron lens 10 defines a central axis 12. A generally curved glass window 14 and an electron sensor 16 are placed along the central axis 12 at a predetermined distance from each other. FIG. 1 shows plural focusing electrodes 18 between the photocathode 14 and the electron sensor 16. Moreover, a photocathode layer 20 is disposed on the glass window. FIG. 1 further shows a first electron trajectory 22, a second electron trajectory 24, a third electron trajectory 26, a fourth electron trajectory 28, a fifth electron trajectory 30, and a sixth electron trajectory 32 within the electron lens 10. It is to be understood that these electron trajectories 22, 24, 26, 28, 30, 32 correspond to an angular spread of ±45 degrees, at an electron energy of 1 eV. It can be appreciated that light 34 can reach the photocathode 14, e.g., from the left looking at FIG. 1 and can cause the photocathode 14 to emit photoelectrons. In turn, the photoelectrons emerging from the photocathode 14 can be focused by the electron lens 18 to the electron sensor 16.
It will be appreciated that non-imaging photosensors based on a reflection-mode photocathode exist, e.g., the non-imaging reflection-mode ReFerence photosensor (see, D. Ferenc, A Novel Photosensor Concept, Nucl. Instr. and Meth. A 471 (2002) p. 229, incorporated herein by reference; D. Ferenc et al., First ReFerence photosensor prototype, Nucl. Instr. and Meth. A 504 (2003) pp. 359-363, incorporated herein by reference). However, so far no imaging phototubes based on a reflection-mode photocathode exist, with the exception of the Blind-HPD in which a blind resembling a Venetian blind is attached to the entrance window of the tube (see, D. Ferenc, Imaging Hybrid Photon Detectors with a Reflective Photocathode, Nucl. Instr. and Meth. A 442 (2000) pp. 150-153, incorporated herein by reference). That design offers good imaging quality, but has only very limited application due to its narrow angular acceptance and wide time jitter.
It will also be appreciated that an imaging electron lens based on a reflection-mode photocathode is problematic in general because the photoelectrons must move in the opposite direction from the incoming light, which requires the focusing electrodes and the position-sensitive electron sensor to reside upstream from the photocathode. This in turn leads to an unacceptable but apparently inevitable optical obstruction of the photocathode.
FIG. 2, for example, shows one such imaging electron lens, generally designated 50, that uses a reflection-mode photocathode. As shown in FIG. 2, the electron lens 50 defines a central axis 52. A non-transparent, generally curved substrate 54 and an electron sensor 56 are placed along the central axis 52 at a predetermined distance from each other. FIG. 2 shows plural focusing electrodes 58 placed between the substrate 54 and the electron sensor 56. Moreover, as shown in FIG. 2, a photocathode layer 60 is disposed on the substrate 54.
FIG. 2 further shows a first electron trajectory 62, a second electron trajectory 64, a third electron trajectory 66, a fourth electron trajectory 68, a fifth electron trajectory 70, and a sixth electron trajectory 72 within the electron lens 50. It can be appreciated that light 74 can reach the photocathode 54, e.g., from the left looking at FIG. 2 and can cause the photocathode 54 to emit photoelectrons. In turn, the photoelectrons emerging from the photocathode 54 can be focused by the electron lens 58 to the electron sensor 56. Unfortunately, it can be quickly appreciated that, in this configuration, the electron sensor 56 and the electron lens 58 block a substantial portion of the light 74 before it reaches the photocathode 54.
Therefore, there is a need for a very high sensitivity (quantum efficiency), single-photon resolution, fast response, minimum dead area, unique constructional simplicity, and extremely low weight camera. The present invention satisfies that need, as well as others.